1. Field of the Invention
The present invention relates to a facsimile apparatus, printer, copier or similar image forming apparatus and more particularly to a color image forming apparatus constructed to sequentially transfer a magenta (M), a cyan (C), a yellow (Y) and a black (BK) toner image to a sheet or similar recording medium being conveyed by a belt with image transfer members one above the other.
2. Description of the Background Art
Color image forming apparatuses extensively used today include the following three types of apparatuses (1) through (3).
(1) Japanese Patent Laid-Open Publication No. 9-50166, for example, discloses an indirect image transfer type of full-color image forming apparatus including a single photoconductive belt or image carrier and developing units each being assigned to a particular color. More specifically, a first developing unit develops a latent image for a first color formed on the photoconductive belt. The resulting toner image of the first color is transferred to an intermediate image transfer belt. Subsequently, a second developing unit develops a latent image for a second color formed on the photoconductive belt, and then the resulting toner image is transferred to the intermediate image transfer belt over the toner image present on the belt. Such a process is repeated in a third and a fourth color. The resulting full-color image is transferred from the intermediate image transfer belt to a sheet.
(2) Japanese Patent Laid-Open Publication No. 10-104898, for example, teaches a direct image transfer type of full-color image forming apparatus including four image forming units each including a respective image carrier. Toner images of different colors are directly transferred from the image carriers to a sheet being conveyed by a belt one above the other. This type of image forming apparatus is generally referred to as a tandem image forming apparatus.
(3) Japanese Patent Laid-Open Publication No. 2001-134042, for example, teaches a tandem, indirect image transfer type of image forming apparatus similar to the above type (2) except that it additionally includes an intermediate image transfer belt. After toner images of different colors formed by the image forming units have been sequentially transferred to the intermediate image transfer belt one above the other, the resulting full-color image is transferred from the belt to a sheet.
The prerequisite with tandem color image forming apparatuses of the types (2) and (3) is that the toner images of different colors be transferred to the sheet or the intermediate image transfer belt in accurate register, i.e., without any color shift.
We proposed in Japanese Patent Application No. 13-0005652 an image forming apparatus including correcting means, or color registering means, for correcting the positional shift of the individual image to be transferred to a sheet. More specifically, a plurality of mark sets each comprising a series of marks of different colors are formed within the circumferential length of the outer surface of a belt. Mark sensing means senses the marks of each mark set formed on the belt. Subsequently, a mean value of the shifts of the marks of the same color included in the mark sets is calculated. Thereafter, the correcting means adjusts, based on the calculated mean values, color-by-color image forming timings assigned to image forming units, thereby correcting the shifts of images to be transferred to a sheet one above the other.
Generally, the belt included in an image forming apparatus of the type described above is passed over a plurality of members including a drive member and tension applying means. The drive member causes the belt to move in a preselected direction while the tension applying means applies tension to the belt. When the drive member is implemented as a drive roller, the belt is caused to move by friction acting between the inner surface of the belt and the surface of the drive roller being rotated. A problem with this type of image forming apparatus is that the belt and drive roller are apt to slip on each other during the conveyance of a sheet. This is because load acting on the drive roller is heavier when the belt conveys a sheet than when it does not convey a sheet. As a result, the linear velocity of the belt is apt to vary between the time when the mark sensing means is sensing the mark sets formed on the belt for the correction of shifts and the time when the belt is conveying a sheet. This eventually brings about the shift of an image on a sheet in spite of the operation of the correcting means.
The slip between the belt and the drive roller or drive member stated above is critical not only in a tandem image forming apparatus but also in any other image forming apparatus so long as it conveys a sheet with a belt.
The tandem full-color image forming apparatus of the type (1) or (2) uses a plurality of image carriers and is therefore feasible for high-speed machines. On the other hand, the full-color image forming apparatus of the type (1) uses a single image carrier and is feasible for machines that attach importance to high image quality. However, in parallel with the spread of personal computers, there is an increasing demand for full-color prints and therefore both of high image quality and high printing speed. In this respect, the full-color image forming apparatus using a single image carrier cannot fully meet the demand for high printing speed due to physical limitations. Therefore, the full-color image forming apparatus using a plurality of image carriers should preferably be configured to implement both of high printing speed and high image quality. While high printing speed is physically easy to achieve with the apparatus including a plurality of image carriers, high image quality is the problem.
Among various factors effecting image quality, the positional shift of a toner image stated earlier is considered to be most difficult to cope with in the full-color image forming apparatus using a plurality of image carriers. This is because any change in the speed of a sheet being conveyed via the consecutive image carriers directly translates into a positional shift, i.e., a color shift.
Further, considering the demand for long-life devices and supplies included in an image forming apparatus, various products each are designed in such a manner as to make the most of the individual characteristic.
In light of the above, Japanese Patent Laid-Open Publication No. 5-134529, for example, proposes to reduce the duration of drive of a developing unit as far as possible by determining whether or not an image is present, thereby extending the life of a developer and that of the developing device. However, the movement of a photoconductive element or image carrier, in many cases, becomes irregular due to the coupling and uncoupling of a clutch assigned to development, as discussed in the above document also. This is apt to bring about color shifts in the case of the full-color image forming apparatus using a plurality of image carriers.
The color shift ascribable to the positional shift is discussed in Laid-Open Publication No. 9-50166 mentioned earlier also. More specifically, adhesion acts between a photoconductive belt and an intermediate image transfer belt due to friction and static electricity. Therefore, if the photoconductive belt and intermediate image transfer belt are different in linear velocity from each other, then one of them pulls the other, resulting in a color shift. Further, adhesion ascribable to static electricity is intensified on the cleaned surfaces of the two belts, but is sharply reduced when toner is present between the belts. In fact, when a developing unit contacts the charged photoconductive belt, toner deposited on background reduces adhesion acting between the two belts.